Seismic surveys are often used by natural resource exploration companies and other entities to create images of subsurface geologic structure. These images are used to determine the optimum places to drill for oil and gas and to plan and monitor enhanced resource recovery programs, among other applications. Seismic surveys may also be used in a variety of contexts outside of oil exploration such as, for example, locating subterranean water and planning road construction.
A seismic survey is normally conducted by placing an array of vibration sensors (accelerometers or velocity sensors called “geophones”) on the ground, typically in a line or in a grid of rectangular or other geometry. Vibrations are created either by explosives or a mechanical device such as a vibrating energy source or a weight drop. Multiple energy sources may be used for some surveys. Additionally, in many surveys the vibrations are created at regular intervals. Moreover, the energy source may be moved to various locations between the vibration events during a seismic survey. The vibrations from the energy source propagate through the earth, taking various paths, refracting and reflecting from discontinuities or “events” in the subsurface, and are detected by the array of vibration sensors. Signals from the sensors are amplified and digitized, either by separate electronics or internally in the case of “digital” sensors. The survey may also be performed passively by recording vibrations in the earth from natural or man made activities.
The digital data from a multiplicity of sensors is eventually recorded on storage media, for example magnetic tape, or magnetic or optical disks, or other memory device, along with related information pertaining to the survey and the energy source. The energy source and/or the active sensors are relocated and the process continued until a multiplicity of seismic records is obtained to comprise a seismic survey. Data from the survey are processed on computers to create the desired information about subsurface geologic structure. Due to differences in propagation time associated with different geophone locations, these processes may involve correlating information received at different geophones such that the data is integrated for better event identification.
In general, as more sensors are used, placed closer together, and/or cover a wider area, the quality of the resulting image will improve. It has become common to use thousands of sensors in a seismic survey stretching over an area measured in square kilometers. Hundreds of kilometers of cables may be laid on the ground and used to connect these sensors. Large numbers of workers, motor vehicles, and helicopters are typically used to deploy and retrieve these cables. Exploration companies would generally prefer to conduct surveys with more sensors located closer together. However, additional sensors require even more cables and further raise the cost of the survey. Economic tradeoffs between the cost of the survey and the number of sensors generally demand compromises in the quality of the survey.
In addition to the logistic costs, cables create reliability problems. Besides normal wear-and-tear from handling, they are often damaged by animals, vehicles, lightning strikes, and other problems. Considerable field time is expended troubleshooting cable problems. The extra logistics effort also adds to the environmental impact of the survey, which, among other things, adds to the cost of a survey or eliminates surveys in some environmentally sensitive areas.
In response to the challenges presented with wired seismic arrays, approaches have been proposed using wireless technologies. For instance, some wireless systems employ a method where digitized data is stored temporarily in a local memory at or near the sensor location. Because the data is not visible to the survey crew, this is often called the “blind read out” method. In these blind read out systems, the seismic data acquired is manually read out from each module once all the seismic data has been gathered. However, such blind read out systems are undesirable, for some survey applications, as any modifications or other problems may not be detected until completion of a survey process. Systems that employ substantially real-time wireless read out have been proposed. One such system is described in U.S. patent application Ser. No. 11/538,744 filed on Oct. 4, 2006 and assigned to Wireless Seismic, Inc., which is hereby incorporated by reference in its entirety. In this system, a wireless read out of an array is accomplished by way of a serial transfer of data between a series of wireless modules. In this regard, a large quantity of data may be automatically retrieved over the serial transfer path between the modules.